text: Cyclodextrins, chitosan, some surfactants, bile acids and salts, sodium tauro-24,25-dihydro-fusidate, and phospholipids were reported as penetration enhancers in nasal drug delivery both for humans and equines. Chitosan is one of the most widely researched penetration enhancers in nasal drug delivery and it enhances the penetration of drugs by opening the tight junctions in the cell membranes.